This invention relates in general to the field of recording media. In particular, one embodiment of this invention provides an erasable optical storage media and write/read/erase mechanism therefor in which data may be recorded and, in some embodiments, erased in response to thermal effects and, in particular in response to light.
Optical data storage media in the form of compact disks are well known as an alternative to longplaying records and magnetic tape cassettes. The disks with which consumers are familiar are optical read-only disks and the common disk player is designed specifically for this type of disk. These disks have a reflective surface containing pits which represent data in binary form. A description of these pits and how they function is provided by Watkinson, "The Art of Digital Audio," Focal Press, Chapter 13.
The pits increase the optical path of the laser beam by an amount equivalent to a half wavelength, thereby producing destructive interference when combined with other (non-shifted) reflected beams. The presence of data thus takes the form of a drop in intensity of the reflected light. The detection system on a standard player is thus designed to require greater than 70% reflection when no destructive interference occurs and a modulation amplitude greater than 30% when data is present. These intensity limits, combined with the focusing parameters, set the criteria for the compact disks and other optical data storage media which can be read or played on such players.
Copending application Ser. No. 294,723 (assigned to the assignee of the present application) describes an improved optical recording method and apparatus. In one embodiment, the invention includes an expansion layer, a reflective layer, and a retention layer. As the expansion layer is heated it expands, pressing into the thin reflective layer and optionally, into, the retention layer which is typically covered by a protective layer. In an alternative embodiment the retention layer is provided between the reflective and expansion layers. In any event the retention layer is pressed into, for example, the protective layer which is sufficiently compliant to allow deformations. The reflective layer is described as, for example, gallium, aluminum, copper, silver, gold, and indium.
While meeting with substantial success, the optical media described above have one or more limitations. For example, two lasers are necessary in some embodiments, each producing separate individual wavelengths: a "recording laser" emitting a beam with a wavelength corresponding to the absorption frequency of a dye in the expansion layer and an "erasing laser" emitting a beam with a wavelength corresponding to the absorption frequency of a dye in the retention layer. Second, manufacture of these media requires several separate coating operations, thereby increasing the risk of defects due to coating flaws, dust and handling, for example. Also, the manufacturing cost is increased with each additional coating operation.
Furthermore, the above optical recording methods and media require that the reflective layer be able to withstand repeated flexures and/or that the adhesion between polymeric layers such as the retention layer be sufficient so as to avoid separation at the interface between the two layers. Further, significant amounts of energy are required to create bumps in a solid or malleable protective layer.
From the above it is seen that an improved optical recording medium and improved methods of writing and erasing data thereon are desired.